1981 V864 Engine BARO MAP Vacuum Connection

[David Weir]

While resolving the issue of numerous error codes the mechanic on my car noticed the following:

The MAP and BARO sensors were connected by vacuum hose with a T off to the manifold vacuum. Has anyone any idea why and what the correct configuration shoud be?

Regards, David Weir

[Tim Pawl CLC#4383]

The MAP sensor should be on its own vac hose from a port in the intake manifold. The BARO sensor located under the dash on the passnger side should have just a little foam sock (filter) on its port.   On cold startup the ECM compares the MAP to the BARO and adjusts the fueling (richens). If the two readings are greater than 5 kpa apart you will trigger a CODE 33 on the error code display. It sounds like someone tried to force the two readings to be closer to the same.  The usual culprit for the readings being greater than 5 kpa apart are  corrosion on connectors (pins), or a poor ground, or just a bad MAP or BARO sensor.  By using the climate control display you can determine which sensor is not reading correctly. With the engine off, sensors should read around 98 kpa (normal baraometric pressure), if one is higher or lower, then clean terminals, determine if a good ground exists (Terminal A) on either wiring harness. If still not corrected, buy a new sensor (MAP is about $60 US from NAPA).

[David Weir]

Hi Tim,Thanks again for the information. Most Appreciated.

[David Weir]

Hi Tim,
 Do you think that the odd vacuum setup would account for the engine running a little rich on starting, even when weather conditions warm, and when cruising at 50/55mph the engine appears to intermitantly hesitate and/or surge briefly?

Regards David Weir

[Mike #19861]

 If the code being set is from the MAP sensor, and it is running rich, the most common cause is a poor vacuum signal to the sensor, other thatn the poor electrical connections Tim mentioned.

 Do a through inspection of the hoses while the engine is running looking for split or collapsed hoses that may give the sensor a low vacuum signal.

 Mike

[Tim Pawl CLC#4383]

Yes, when the initial key on check of the MAP is lower than the BARO the ECM richens. The initial check is before the engine starts, so it doesnt matter if outside temperature is cold or warm. The only time it doesnt do the cold start check is when the engine coolant temp is already warm. i.e. Any hot restart.  Why your car is hesitating at 50/55 mph is  probably not related to MAP/BARO, unless there is a vacuuum leak. It is more probably a point where the number of cylinders is trying to modulate between 6 and 4 or 8 and 4 (highway speeds) and it is leaning out the mixture.

[David Weir]

Hi,
The saga continues:
After leaving the car for 24 hours I checked the MAP and BARO pressure readings. 2 was 97, 3 was 99.

I started the car and it ran fine for about 45 seconds or so before it started to start running rough, smoking and a little soot? comming from the exhaust. It did at one time stall.

I turned the car off and an Engine code 35 had been set.

Checking the MAP and BARO readings again both 2 and 3 were 99.


I did at one stage manage to read the MAP and BARO pressures when the car was running and got 2 as 38 and 3 as 97. I am not sure how relevant this is but it seems wrong.

Any ideas?

[Tim Pawl CLC#4383]

Your MAP and BARO are operating properly they should be within 5 kpa with car off. With car on ,the MAP is reading manifold vacuum and should be a lower number. I will check my diagnostics book for CODE 35 and get back to you.

[Tim Pawl CLC#4383]

Codes 31 and 35 are basically the same failure , Code 31 is the MAP sensor and Code 35 is the BARO sensor. Since you got both codes at the same time, the suspicion is that you have a wiring harness issue going to the ECM, or a bad ECM.   The suggestion is to set your monitor of the sensors #2 and #3, and watch the values for the MAP and BARO as you wiggle the connector to the ECM....if the values change it is in the connection. The other possibility is a bad ground from the ECM to the dash frame.  Terminal A on both the MAP and BARO sensors is the ground wire to the ECM.

[David Weir]

Thank you for the reply, I will investigate further. Regards David